As an RFID system, there exist an active type system in which a tag itself accommodates a power supply integrally, and makes origination of an identification signal (ID), and a passive type system in which after a tag receives the power supply from a reading device (reader), it gives an ID as a reply. In particular, it is expected that the system employing a UHF band as a radio frequency, out of the passive type RFID systems, is put to practical use as a system for inspecting a plurality of commodities in a lump in a distribution spot, or the like because it has a longer read range performance as compared with the system employing another frequency band. Specifically, the tag, being a transmitting device having a function of transmitting an inherent ID, is affixed to each commodity, and a forklift carrying a plurality of commodities passes through a reading gate (reader gate) equipped with a read antenna, thereby allowing an inspection as to which commodity is received/delivered to be automatically made.
Needless to say, such a practical use form demands a level close to 100% as a probability (a read rate) that the ID being transmitted from the tag is correctly read off; however as a matter of fact, the read rate declines due to various factors. As one factor for causing this read rate to decline, the fact that a collision avoidance protocol does not operate efficiently under an environment in which a reception power of the tag fluctuates due to a fading can be listed. This phenomenon will be explained in details.
When the reader requests an ID reply of a plurality of tags, each of these tags transmits the ID as a reply at an identical timing after receiving a command from the reader. For this, reply signals of the tags collide with each other, and resultantly the ID cannot be read off in the reader side. So as to avoid a collision of the tag companions, the passive type RFID system is provided with a collision avoidance protocol. In the collision avoidance protocol, the tags, each of which becomes an object of the reading-off, are grouped responding to the IDs, and each group is caused to individually giving the ID as a reply in some cases, or the tag, which has been recognized once by the reader, is caused to stop to give the ID as a reply for a constant time period in some cases, thereby avoiding a collision of the tag companions
On the other hand, in an actual operation environment, the fading occurs because a plurality of propagation paths exist between an antenna of a reader and a tag, and resultantly they overlap each other. For this, the reception power from the reader largely fluctuates while the tag passes through the reader gate, so the case that the power supply is switched on owing to supplying a sufficient power to the tag, and the case that the power supply is switched off due to supplying an insufficient power to the tag alternate. In a case where the power supply is switched off when the tag is receiving a command from the reader, or when the tag itself is transmitting the ID as a reply, it follows that the ID transmission from the tag to the reader is not made correctly.
Further, the passive RFID tag is provided with a parameter representative of a “status”. As the so-called “status” of the tag, there exist, for example, a status (standby status) in which the tag stands by for a purpose of making the ID reply, a status (reply stop status) in which the ID has been already read off by the reader and the ID reply has been stopped, or the like. In the case that after the power supply of the tag is switched off temporarily, it is switched on again, these statuses cannot be stored as a status prior to the switching-off of the power supply. For this reason, when after the power supply of the tag, which is in a reply stop status, is switched off temporarily, it is switched on again, the above tag makes the ID reply again because it has not stored that it was previously in a reply stop status, which causes a collision to occur. Such a phenomenon causes the collision avoidance protocol not to operate efficiently in the actual operation environment, and hence the read rate to decline.
Alleviating such a problem of a decline in the read rate necessitates a design scheme such as optimizing an installation location of the antenna, and an operation schemes such as putting limits to the number of the tag and slowing the migration speed. Obtaining such a design guideline and an operation guideline, however, necessitates the method of precisely estimating the read situation (read rate etc.) of the tag in the actual operation environment, and its system.
As a related art of the simulator of estimating a performance in the RFID, there exists the technique disclosed in Non-Patent document 1. This technique is a technique of estimating a radio wave propagation situation ranging from the active type RFID tag to the receiver with a radio wave propagation estimating method, and visualizing the readable range. Applying this technique for the passive RFID makes it possible to visualize the range in which the ID can be read off by the reader antenna; however, as a matter of fact, how many pieces of the tags can be read off correctly cannot be estimated because plural pieces of the tags pass through its readable range.
Further, as a technique of analyzing the upper limit of the migration speed of the tag from a result of having actually measured the fading and an operation of the collision avoidance protocol, there exists the technique disclosed in Non-Patent document 2. The Non-Patent document 2, in which 1.3 sec. is calculated as a time necessary for reading off the 60 pieces of the tags in a lump in the collision avoidance protocol, mentions that it is difficult to secure an endurance time (a time that the reception power equal to or more than a reception sensitivity has been secured for the tag) of 1.3 sec. judging from the situation of the fading actually measured. Employing the technique disclosed in this document makes it possible to calculate back to the migration speed for securing the endurance time of 1.3 sec., which can be adopted as an operation guideline. However, this technique is grounded upon the assumption that all tags are fogged with an identical fading pattern, respectively. That is, it is grounded upon the assumption that all tags are in a status of having been switched on during the endurance time of 1.3 sec.
However, it is unthinkable that all tags are fogged with an identical fading pattern, respectively, in the actual operation environment because the situation in which the fading occurs changes when a space between the tag companions becomes equal to or larger than a half of a wavelength (with a UHF band RFID, approx. 16 cm. That is, even though it is supposed that the endurance time of 1.3 sec. has been secured for one certain piece of the tag, all tags cannot be read off always for 1.3 sec. because any of the other 59 pieces of the tags might be in a status in which the power supply thereof has been switched off. Contrarily, when the endurance time of a certain extent has been intermittently secured for all tags even though the endurance time of 1.3 sec. has not been secured, the case as well that all tags can be read off could occur. With the technique described in this non-patent document 2, it is impossible to cope with such a situation that occurs in the actual operation environment.
On the other hand, upon paying an attention to the method of estimating a performance in the communication system other than the RFID system, as a technique of estimating a throughput feature with a co-simulation of a physical layer and an MAC layer, there exists the technique disclosed in JP-P2001-168904A (Patent document 1). This technique is a technique of preparing snapshots of working spaces of a plurality of users, selecting one part of packets generated in the above working spaces as a loss packet based upon a result of estimating the radio wave situation with a physical layer simulation, processing the remaining packets not selected as a loss packet with an MAC layer simulation, and estimating a throughput feature.
Now think whether this technique described in this Patent document 1 can be applied for estimating the read rate of the passive RFID by replacing the user with the tag, the packet with the sending-out of the ID from the tag, the physical layer simulation with the estimation of the radio wave situation, the MAC layer simulation with the collision avoidance protocol, and the throughput with the read rate, respectively.
With the technique described in this patent document 1, it follows that the positions of plural pieces of the tags are secured as one snapshot, the tags that cannot give the ID as a reply are pre-excluded based upon an estimation result of the radio wave situation, the collision avoidance protocol is applied only for the tags that have not been excluded. That is, the physical layer simulation (exclusion of the tags based upon the radio wave situation estimation) and the MAC layer simulation (collision avoidance protocol) are executed separately. At the moment of estimating the read situation of the passive RFID, however, the power supply on/off situation of the tag in the middle of a command of the foregoing collision avoidance protocol being processed in the tag largely exerts an influence upon an operation efficiency of the collision avoidance protocol.
Thus, the collision avoidance protocol and the radio wave situation need to be synchronized with each other for analysis. That is, the operation of the tag for the command of the collision avoidance protocol has to be appropriately changed according to the power supply on/off situation of the tag in the middle of the above command being processed in the tag. For this reason, this technique of separately executing the physical layer simulation and the MAC layer simulation for one snapshot is not applicable.
Further, at the moment of estimating the read situation of the passive RFID, a change in the status of the tag due to the foregoing power supply on/off situation of the tag largely exerts an influence upon an operation efficiency of the collision avoidance protocol. Thus, it is necessary that each tag holds a parameter representative of the “status” and the status of this tag is synchronized with the collision avoidance protocol and the radio wave situation for analysis. That is, the status of the tag has to be appropriately changed according to the power supply on/off situation of the tag in the middle of the command of collision avoidance protocol being processed in the tag. For this reason, this technique described in the Patent document 1, which includes no parameter equivalent to the status of the tag, is not applicable.    [Non-Patent document 1] Hiroshi Sugawara, Takeshi Ono et al.; “RADIOSCAPE-RFID”, a visual simulator for communication qualities of active-RFID systems;” Proceedings of the 2005 IEICE General Conference, B-5-113    [Non-Patent document 2] Jin MITSUGU and Hisakazu HADA; “Performance degradation of 950 MHz RFID due to a fading;” Proceedings of the 2005 IEICE General Conference, B-1-38    [Patent document 1] JP-P2001-168904A